There is an increased need to remove harmful substances such as fine particles contained in an exhaust gas emitted from an internal combustion engine, a boiler or the like in consideration of the impact on the environment. In particular, regulations concerning removal of graphite fine particles (hereinafter, referred to as “PM”) emitted from the diesel engine tend to be tightened in Europe and the United States as well as in Japan, so that attention is being given to use of a honeycomb structure in a collecting filter for removal of PM (hereinafter, referred to as “DPF: Diesel Particulate Filter”) and an exhaust emission control system for an internal combustion engine using the honeycomb structure.
In general, as shown in FIG. 12, the honeycomb structure for use in this purpose has a number of cells 3 partitioned from one another by partition walls 2. The cells extend through an X-axis direction. The honeycomb structure has a structure in which the cells 3 adjacent to each other are plugged in one end on an opposite side alternately so that end surfaces have checkered patterns. In such a honeycomb structure, subject fluid flows in the cell 3 not plugged at inflow end face 42, that is, plugged at outflow end face 44, passes through the porous partition walls 2, and is discharged via the adjacent cells 3, that is, the cell 3 plugged at the inflow end face 42 and not plugged at outflow end face 44. In this case, the partition walls 2 act as a filter. For example, PM discharged from a diesel engine is trapped by the partition walls and deposited on the partition walls.
The exhaust emission control system for an internal combustion engine that employs the filter of the aforementioned type as an emission control device suffers from pressure loss of exhaust gas. That is, pressure loss of exhaust gas occurs across the honeycomb structure, causing decrease in output of the internal combustion engine and deterioration in fuel consumption. For solving this problem, although it is useful to increase the porosity of the honeycomb structure for reducing the pressure loss, increase in porosity of the honeycomb structure usually causes decrease in mechanical strength of the honeycomb structure. Therefore, there is a need for a honeycomb structure and an exhaust emission control system using the same which is low in pressure loss and good in strength.